Normally, the conventional ladder is designed for standing on an even ground. However, it is frequent that an uneven ground such as steps of a stair or slope land is met while the ladder is in use. As a result, the conventional ladder becomes lacking stability to impair personal safety of the user.
U.S. Pat. No. 5,074,377, which is corresponding to German Patent Application No. G8814195), discloses a jointed ladder with a locking mechanism for the ladder being adjusted to different operation heights. The reference proposes a pressure plate (28) with a slotted hole (33) and a bolt (27) with a knob (30) (FIG. 6); the extension stay (16) can be moved relative to the pressure plate (28) or fixed at a desired place by means of loosening or tightening the bolt (27). A friction force between the pressure plate (28) and the extension stay (16) plays an important role to keep the extension stay in place in addition to the fastening force of the bolt (27). However, to hold the extension stay with the friction force has a deficiency of slipping out and it is an unreliable structure.